configure: Fix compiler warning in config.log (macro redefined)

warning: "_GNU_SOURCE" redefined

The macro is already defined on the command line.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -2251,7 +2251,6 @@ fi
 # signalfd probe
 signalfd="no"
 cat > $TMPC << EOF
-#define _GNU_SOURCE
 #include <unistd.h>
 #include <sys/syscall.h>
 #include <signal.h>